A Program Project will be established in order to support an interactive program of research dedicated to understanding the molecular mechanisms of the cell interactions that control development. The cell interactions controlling cell proliferation will be investigated by a molecular analysis of a cell-proliferation control gene in Drosophila which encodes a cell adhesion molecule, and by studying the relationship between adhesion and proliferation of imaginal disc cells in vitro using a new culture system. Several genes required for the cell interactions that control cell and tissue polarity will be investigated at the genetic and molecular levels in Drosophila. Genes involved in the cell interactions governing head and foot formation in hydra will be isolated and characterized, their expression patterns studied, and their functions investigated in transgenic animals. The Project will support a Research Core Unit for antibody production, confocal microscopy, and image analysis which will be used by all of the component projects. The work will contribute to our understanding of the molecular mechanisms controlling normal development and regeneration as well as the defects in cell interaction mechanisms that underlie birth defects and cancer.